1. Field of the Invention
The present invention relates to the field of laser processing methods and systems, and specifically, to laser processing methods and systems for processing microstructures formed on substrates. This invention is particularly applicable, but not limited to, laser repair of redundant semiconductor memory devices.
2. Background Art
In the repair of memory integrated circuits such as DRAMs and laser programming of high-density logic devices, the use of new materials, such as aluminum, gold, and copper, coupled with the small geometry of these devices, make the problem of link removal difficult. The new materials are typically metals or highly conductive composites having reflectivity that is well over 90% in the visible and near infrared wavelength regions. Aluminum, for example, reflects greater than 90% of the laser energy over the range from the UV through to the near infrared. Gold and copper reflects even more strongly in the near infrared, the wavelengths of choice used by most of the lasers repairing memories in production.
Further, economics and device performance have driven the size for the DRAMs and logic devices to very small physical dimensions. Not only are the devices small, but the interconnects and links thickness have also decreased dramatically in recent years.
Thermal laser processing of links relies on the differential thermal expansion between the oxide above the link and the link itself. This differential expansion results in a high pressure build-up of the molten link contained by the oxide. The oxide over the link is necessary to contain the link in a molten state long enough to build-up sufficient pressure to crack the oxide and explosively expel the link material. If the pressure is too low, the link will not be removed cleanly. Alternative laser wavelengths and laser control strive to increase the laser “energy window” without damaging the substrate and material contiguous to the link.
Descriptions of an all-copper, dual-Damascene process technology can be found in “Benefits of Copper—Copper Technology is Here Today in Working Devices,” NOVELLUS DAMASEUS, Dec. 20, 2001; and “Preventing Cross-Contamination Caused By Copper Diffusion and Other Sources,” P. Cacouvis, MICRO, July 1999.
FIGS. 2a and 2b illustrate prior art laser processing of multi-layer structure wherein a target structure is located in proximity to a substrate, with a q-switched pulse 20 from a conventional solid state laser 21 irradiating and overfilling a target structure 23. A laser spot size is typically significantly larger than the (target) link size which relaxes precision positioning requirements. A laser wavelength is typically selected based on substrate 27 (commonly Silicon) transmission so as to allow for higher peak laser power or other system and process variations. In certain cases, a layer 28, 25 absorption coefficient is controlled (e.g., as a transition or protective layer) and/or a wavelength selected wherein substrate damage is avoided.
Further information is available regarding link blowing methods and systems, including material processing, system design, and device design considerations, in the following representative U.S. patents and published U.S. applications: U.S. Pat. Nos. 4,399,345; 4,532,402; 4,826,785; 4,935,801; 5,059,764; 5,208,437; 5,265,114; 5,473,624; 6,057,180; 6,172,325; 6,191,486; 6,239,406; 2002-0003130; and 2002-0005396.
Other representative publications providing background on link processing of memory circuits or similar laser processing applications include: “Laser Adjustment of Linear Monolithic Circuits,” Litwin and Smart, ICAELO, (1983); “Computer Simulation of Target Link Explosion In Laser Programmable Memory,” Scarfone, Chlipala (1986); “Precision Laser Micromachining,” Boogard, SPIE Vol. 611 (1986); “Laser Processing for Application Specific Integrated Circuits (asics),” SPIE Vol. 774, Smart (1987); “Xenon Laser Repairs Liquid Crystal Displays,” Waters, Laser and Optronics, (1988); “Laser Beam Processing and Wafer Scale Integration,” Cohen (1988); “Optimization of Memory Redundancy Link Processing,” Sun, Harris, Swenson, Hutchens, Vol. SPIE 2636, (1995); “Analysis of Laser Metal Cut Energy Process Window,” Bernstein, Lee, Yang, Dahmas, IEEE Trans. On Semicond. Manufact., Vol 13, No. 2. (2000).
Also, the following co-pending U.S. applications and issued patents are assigned to the assignee of the present invention and are hereby incorporated by reference in their entirety:                1. U.S. Pat. No. 5,300,756, entitled “Method and System for Severing Integrated-Circuit Connection Paths by a Phase Plate Adjusted Laser beam”;        2. U.S. Pat. No. 6,144,118, entitled “High Speed Precision Positioning Apparatus”;        3. U.S. Pat. No. 6,181,728, entitled “Controlling Laser Polarization”;        4. U.S. Pat. No. 5,998,759, entitled “Laser Processing”;        5. U.S. Pat. No. 6,281,471, entitled “Energy Efficient, Laser-Based Method and System for Processing Target Material”;        6. U.S. Pat. No. 6,340,806, entitled “Energy-Efficient Method and System for Processing Target Material Using an Amplified, Wavelength-Shifted Pulse Train”;        7. U.S. Ser. No. 09/572,925, entitled “Method and System For Precisely Positioning A Waist of A Material-Processing Laser Beam To Process Microstructures Within A Laser-Processing Site”, filed 16 May 2000, and published as WO 0187534 A2, December, 2001;        8. U.S. Pat. No. 6,300,590, entitled “Laser Processing”; and        9. U.S. Pat. No. 6,339,604, entitled “Pulse Control in Laser Systems.”        
However, it is to be understood that this listing is not an admission that any of the above references are prior art under the Patent Statute.
The subject matter of the above referenced applications and patents is related to the present invention. References to the above patents and applications are cited by reference number in the following sections.